1. Field of the Invention
The present invention relates to devices for amplifying an input signal. Specifically, the present invention relates to using amplifier devices in pulse echo apparatus or in ultrasonic apparatus.
2. Discussion of the Related Art
In terms of the present invention, a pulse echo apparatus can be in the form of a radar apparatus, a sonar apparatus or an ultrasonic apparatus, wherein a transmission signal having a very high signal amplitude is radiated via a correspondingly designed converter into a medium to be examined, such as an air monitoring space in the case of the radar apparatus, a water monitoring space in the case of the sonar apparatus, or a human body in the case of the ultrasonic apparatus. A reception signal produced by reflection in the medium to be examined is detected and evaluated after corresponding amplification. Such reception signal may form the input signal of the amplifier device.
In the case of an ultrasonic apparatus, a reception signal attributable to near-surface reflection in a human body to be examined has a relatively high signal amplitude. By contrast, on account of a high degree of tissue attenuation, an input signal produced by reflection from a deeper tissue layer has a very small signal amplitude. In order that a region lying deep in the tissue can still be imaged well, good noise behavior is demanded of an amplifier device. This means that the amplifier device should be embodied in a low-noise and noise adapted manner.
U.S. Pat. No. 5,879,303 discloses a specific imaging method for an ultrasonic apparatus. In this so-called THI (Tissue Harmonic Imaging) method, the first harmonic of the reception signal is evaluated. Non-linearity in human tissue gives rise to this first harmonic (second harmonic component) of the fundamental frequency of the transmission signal that is radiated in. Therefore, an amplifier device used in this connection should be designed such that it has very little distortion in order that the measurement result is not corrupted by an additional harmonic component produced in the amplifier device.
Joachim von Parpart's “Breitbandige Ferrit-Hochfrequenztrans formatoren” [Broadband Ferrite Radio frequency Transformers], Huthig Verlag Heidelberg, 1997, pages 130 and 131, discloses that practically no second-order distortion, i.e. no first harmonic of the fundamental frequency, arises in a push-pull amplifier. In this case, components of the input signal having different polarity are amplified separately in two structurally identical individual amplifiers and combined again at the output to form a common output signal. While the input signal is amplified, even-order distortions, including the first harmonic, largely average out during push-pull amplification. Thus, low-distortion amplification is achieved.
Furthermore, the Siemens-Datenbuch “Individual Semiconductors, Discrete and RF Semiconductors”, second edition, 1997, pages 1073 to 1078, discloses that an amplifier device comprising a bipolar transistor as a semiconductor amplifier operates in a particularly low-distortion manner if a high collector quiescent current is provided. This places an operating point into a region of the transistor characteristic curve in which the transistor has a practically linear behavior. As the collector quiescent current rises, the characteristic curve linearity rises and so does the degree of freedom from distortion that can be achieved with the amplifier device.
However, the low-distortion amplifier devices mentioned do not afford optimization with regard to noise. Moreover, a high collector quiescent current favorable for offering freedom from distortion leads, over high static power loss.
U. Tietze, Ch. Schenk, “Halbleiter-Schaltungstechnik” [Semiconductor Circuitry], Springer-Verlag, 9th edition, 1991, pages 78 to 82 describes that the noise behavior of a transistor depends to a critical extent on the collector quiescent current. In particular, the inherent noise of the transistor has a minimum at a predetermined collector quiescent current. The transistor noise deteriorates again at a higher collector quiescent current.
Consequently, the known amplifier devices are optimized either with regard to their noise behavior or with regard to their distortion behavior.